Britain is to expel 90 Soviet diplomats who have been engaged in active espionage, the Foreign Office announced last night. Another 15 Soviet officials, at present overseas, will not be allowed to return to this country. Many of these men are suspected of involvement in planning acts of sabotage. The expulsion order – affecting nearly 20 per cent of the 550 Soviet diplomats in Britain – is unprecedented in size and scope. It follows months of intensive investigation by the intelligence services, and the defection of a top KGB officer from the Soviet Embassy in London.

The KGB man, who had the rank of major, proved the catalyst for the "clearing" operation against Soviet espionage. He gave the security services a comprehensive breakdown of his country's espionage apparatus in Britain – and also supplied details " … of plans for infiltration of agents for the purposes of sabotage", the Foreign Office said. In the opinion of Mr Heath and Sir Alec Douglas-Home this crisis over diplomatic espionage by Soviet officials is so serious that the British Government cannot, and will not, enter into preparations for the European Security Conference proposed by the Soviet Union until the crisis is resolved.

The terms of the expulsion of the Soviet diplomats in London were spelt out in a terse, toughly worded Aide Mémoire. The 90 diplomats have been given two weeks to leave Britain. From now on, "the numbers of Soviet officials in the various categories … will be limited to the level at which they stand after the withdrawal of the persons referred to and if a Soviet official is required to leave the country in future as a result of his having been detected in intelligence activities, the ceiling in that category will be reduced by one".

A lengthy – and equally strong-worded – Foreign Office statement recalled that the size of the Soviet Embassy was limited in November 1968 "but the numbers in other categories continued to grow. The total is now over 550, which is higher than the comparable figure for Soviet officials appointed to any other Western country, including the United Slates".

Much of the statement was devoted to the role of the KGB agent who defected a few weeks ago. "This man, an officer of the KGB, brought with him certain information and documents, including plans for infiltration of agents for the purpose of sabotage." Agents of all British security services – MI5, MI6, MI7 and the Special Branch – have taken part in the operation against the Soviet spy rings spread over the past nine months.